Investigation of electron-induced cross-linking of self-assembled monolayers by scanning tunneling microscopy

Ultrathin membranes with subnanometer pores enabling molecular size-selective separation were generated on surfaces via electron-induced cross-linking of self-assembled monolayers (SAMs). The evolution of p-terphenylthiol (TPT) SAMs on Au(111) surfaces into cross-linked monolayers was observed with a scanning tunneling microscope. As the irradiation dose was increased, the cross-linked regions continued to grow and a large number of subnanometer voids appeared. Their equivalent diameter is 0.5 ± 0.2 nm and the areal density is ≈1.7 × 1017 m−2. Supported by classical molecular dynamics simulations, we propose that these voids may correspond to free volumes inside a cross-linked monolayer.

The spatial distribution of the dark spots was evaluated by dividing the STM image into equal sections and then counting the number of spots in each section. The spatial distribution can be approximated by a Poisson distribution, indicating that the dark spots are random and independent of each other. Figure S6: Simulated cross-linked (in black) and non-cross-linked (in orange) molecules according to the step-growth model. The area fractions are 3%, 17%, and 30%, respectively. The size distribution of reacted or cross-linked molecules obtained by simulation are plotted in comparison to the size distribution of the dark spots (also see Figure 3h in the main manuscript) observed by STM. The area in STM imaging is 29700 nm 2 and the simulated data is multiplied by a factor of 29700/6400. One pixel represents one molecule.

Expected areal number density of dark spots in the chain-growth scenario [1]
According to Amiaud et al. [2], cross-linking within a TPT SAM may proceed via radical chain reactions starting with a resonant electron attachment process at 6 eV. The formation of the first radicals initiating chain reactions each may then proceed via electronic rearrangement or via dissociative electron attachment (DEA). As the irradiation was performed with 6 eV primary electrons, that is, below ionization and excitation thresholds, the other (non-resonant) scattering processes, which lead to the formation of the first radicalized monomers such as neutral dissociation (ND) or dissociative ionization (DI), could be excluded. TPT on Au(111) may have the first ionization potential between 6 and 9 eV. However, as our irradiation experiments were performed with 50 eV and 1 keV primary electrons, all electron-induced fragmentation pathways need to be taken into account in the first place.
It is assumed in the following that the dark spots observed in the STM images are formed upon generation of one (first) radical each of which subsequently initiates radical chain reactions. To investigate the potential contribution of the emitted 6 eV secondary electrons (SE) to the creation of the first radicals, the areal number density of the dark spots observed in the STM image shown in Figure S4b, , is compared with the expected areal number density of reactive electron attachment (EA) events, .
Reactive EA events are defined as EA events that eventually lead to the formation of at denotes the areal number density of secondary electrons (SE) emitted within the window of the resonance, that is, with kinetic energies of 6.0 ± 1.5 eV. ,6 can be determined by employing the following equation: is estimated to be 4.1 ± 1.7 × 10 14 cm −2 .
Therefore, the expected areal number density of reactive EA events, upon 1 keV electron exposure with a dose of 0.5 mC/cm 2 , amounts to = 1.7 ± 0.7 × 10 13 cm −2 . This value is to be compared to the areal number density of the dark spots observed in the STM image shown in Figure S4b, which amounts to = 2.0 ± 1.0 × 10 12 cm −2 . By contrasting the areal number densities it is seen that is lower than by roughly one order of magnitude. This result appears counterintuitive as was derived from the reactive EA cross-section determined on the basis of HREELS data [2]. Assuming that each dark spot is created upon the formation of a TPT radical monomer, is expected S7 to be equal to or higher than , in particular, as further reaction pathways (ND and/or DI) may contribute to the formation of radical monomers at higher energies [5].
However, the reactive EA cross-section was overestimated by Amiaud et al.
HREELS data reveals that a 6 eV electron irradiation with a dose of 50 electrons per molecule leads to a decrease of 47-53% of the aromatic CH stretching feature. As one TPT monomer has 13 aromatic CH groups, 6-7 aromatic carbon centers, on average, are converted into aliphatic carbon centers after irradiation. Without considering the propagation of radical chain reactions, that is, only the reaction between two monomers is taken into account, the creation of one radical center causes the formation of two aliphatic groups. Provided that every DEA event leads to a reaction with an adjacent monomer, three DEA events per monomer are required, on average, to cause the observed ≈50% loss of aromaticity. Considering that every monomer is irradiated by 50 electrons and occupies an area of ≈20 Å², the reactive EA cross-section is to the case when the propagation is neglected. Therefore, the (over)estimated reactive EA cross-section ~ 1.2 × 10 −16 cm 2 is to be divided by (n − 1). The STM data indicates that, on average, 5-6 monomers are involved in the radical chain reactions.
Considering the propagation of radical chain reactions with n = 5 -6 monomers involved, the reactive EA cross-section is reduced to ≈ 1.2 × 10 −16 cm 2 /(n − 1) ≈ 2.2 ± 0.3 × 10 −17 cm 2 . Therefore, this allows for the estimation of the expected areal number density of reactive EA events upon 1 keV electron exposure with a dose of 0.5 mC/cm 2 , which is = 3.8 ± 1.9 × 10 12 cm −2 . When contrasting this value with = 2.0 ± 1.0 × 10 12 cm −2 , the areal number density of the dark spots observed in the STM image shown in Figure S4b is in good agreement with the expected areal number density of reactive EA events when considering radical chain reactions.